Tracking down that kind of evidence in a war zone is nearly impossible.

"The gold standard would be samples from a munition or fragment of a munition that tested positive for a known military chemical agent," said Gregory Koblentz, a chemical weapons expert at the Council on Foreign Relations.

"The best thing is to have an investigation team go to a site," said Ralf Trapp, a chemical weapons consultant based in France.

U.N. inspectors have not been allowed into the country.

It is likely that the United States and allied intelligence officials are relying on soil samples or other physical evidence that was gathered at a site and then smuggled out of the country, possibly by opposition groups, analysts say.

That raises its own set of problems.

"The problem you face is how do you demonstrate that the sample you have came from the alleged incident, in other words, that it is authentic and it hasn't been tampered with," Trapp said.

The White House said Thursday it assesses "with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale" in Syria.

The Obama administration was careful to qualify its assessment, saying it is based "in part on physiological samples" and further corroboration is required. It is not clear whether the physiological sample was referring to soil samples or other physical evidence gathered in Syria.

"For example, the chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions," the White House said in a letter to lawmakers.

The White House assessment follows similar statements by Great Britain, France and Israel.

Brig. Gen. Itai Brun, the head of research and analysis in Israeli military intelligence, referred to victims' symptoms when he accused Syria of using chemical weapons.

"Shrunken pupils, foaming at the mouth and other signs indicate, in our view, that lethal chemical weapons were used," he said.

The flawed intelligence that helped launch the Iraq War has made the Obama administration more wary of placing too much faith in intelligence. In letters to Sens. Carl Levin, D-Mich., and John McCain, R-Ariz., the White House said "intelligence assessments alone are not sufficient."

"The Iraq WMD fiasco is a cautionary tale here," Koblentz said.

The White House assessment said the regime's use of chemical weapons was on a "small scale," making it even more challenging to gather proof, analysts say.

Large-scale attacks, such as the gassing of Kurds in Halabja, Iraq, in 1988, are hard to hide because they produce mass casualties, Trapp said. Evidence usually includes video, eyewitness accounts and other details.

Analysts are also puzzled over the motivation for limited use of chemical weapons, which are usually used on a large scale to shock and terrorize an opposing army or civilian population. Using chemicals on a small scale would risk further political alienation without achieving anything militarily, which "doesn't make that much sense," Koblentz said.

It's possible the weapons fell into the hands of a rogue commander, but that raises question of whether some of the stockpiles remain under under Assad's control, analysts said.